Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine in part due to the engine being shut down during times when the engine operates inefficiently, or is not otherwise needed to propel the vehicle. During these conditions, the vehicle is transitioned from an engine mode to an electric mode where the electric motor is used to provide all of the power needed to propel the vehicle. When the driver demand for power increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine is restarted. Vehicle propulsion is then transitioned from an electric mode to an engine mode.
One method of enabling a smooth engine restart in an HEV powertrain is disclosed by Tulpule et al. in US 20140088805. Therein, a disconnect clutch is disposed between an engine and a motor, which is operable to disconnect the engine from the motor. During an engine restart, the disconnect clutch is disengaged so that the engine can be fueled to obtain a speed that matches the motor speed. Then, when the engine speed matches the motor speed, the disconnect clutch is engaged to couple the engine and the motor to the drive shaft to meet the driver torque demand. In another example disclosed by Sah et al. in US 20120323418, engine speed and transmission input speed is synchronized when an oncoming clutch is activated and an outgoing clutch is deactivated.
However the inventors have recognized potential issues with such an approach. If a driver demands torque during the engine starting, it may be challenging to provide sufficient torque (e.g., from the motor) to meet the driver demand as well as start the engine. If motor torque is used to meet the driver demand, engine starting may be delayed, degrading engine acceleration. The engine performance may be particularly slow when the vehicle driver tips in heavily to launch the vehicle from a stationary state. If motor torque is used to start the engine, the driver demand may not be met in a timely manner, degrading the operator's drive experience. In addition, due to the continuous change in motor speed resulting from the variation in operator torque demand, it may be difficult for an engine controller to predict a target synchronous motor speed at which to close the disconnect clutch and enable the transition from electric mode to hybrid mode.
The inventors have recognized these issues and developed a method for a hybrid vehicle with an improved engine restart method. In one example, a driveline method comprises: closing a driveline disconnect clutch to start an engine; and adjusting engine torque after a first combustion event in response to each of accelerator pedal position and speed of a motor. In this way, engine speed can be controlled during an engine run-up based on an accelerator pedal position to expedite engine restart and enable an earlier transition to vehicle propulsion using engine torque.
As an example, while a vehicle is propelled via motor torque from an electric motor, an engine restart request may be received. The engine restart may be requested responsive to a heavy accelerator pedal tip-in by the vehicle operator. Accordingly, the engine may enter a first phase of an engine restart operation wherein the engine is cranked, unfueled, via the electric motor with a disconnect clutch coupled between the engine and motor at least partially open. The torque capacity of the disconnect clutch may be adjusted during the first phase so as to provide sufficient torque to overcome the compression braking torque of the engine and crank the engine. The motor torque may be transiently raised during the cranking so as to provide sufficient torque for engine cranking and concurrent vehicle propulsion. Following cranking, the engine may enter a second phase of the restart operation where engine fueling and cylinder combustion is resumed, and the engine speed is controlled based on the motor speed and the accelerator pedal position to provide the maximum possible torque under the given operating conditions. Herein, the engine speed is controlled to a target synchronous speed that corresponds to a current motor speed, or a predicted motor speed at a time of disconnect clutch engagement. In addition, the engine speed is further controlled based on the accelerator pedal position (or driver torque demand) so as to provide maximal engine torque at the given engine speed to accelerate the engine to the target synchronous speed as rapidly as possible, and to concurrently prepare the engine air path for torque delivery after the engine connects to the driveline. The engine speed is controlled using adjustments to engine fueling, spark timing, and throttle position while the disconnect clutch is maintained partially disconnected. When the engine speed approaches the motor speed (while remaining below it), the engine enters a third phase of the restart operation wherein the disconnect clutch is gradually engaged. Therein, the engine torque is reduced below the disconnect clutch capacity so that the engine can continue to accelerate to the target speed without overshooting the target speed. When the engine speed matches the synchronous speed, the disconnect clutch may be closed and the vehicle may be transitioned into being propelled using engine torque only.
In this way, a quality of engine restarts in a hybrid electric vehicle, such as those performed while an operator is tipping in, may be improved. By controlling the engine speed during a run-up phase of an engine restart based at least on an accelerator pedal position, the engine speed can be raised to a target synchronous speed faster while the engine's air path is better prepared for elevated engine torque delivery following disconnect clutch engagement. By transiently lowering an instantaneous engine torque prior to, and during, disconnect clutch engagement, NVH issues and driveline torque disturbances associated with overshooting (or undershooting) of the target synchronous speed can be reduced. Overall, a faster engine restart and transition into vehicle propulsion using engine torque is enabled, allowed for expedited engine acceleration and vehicle launch.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.